Constructing and understanding Bayesian networks for legal evidence with scenario schemes

Abstract

In a criminal trial, a judge or jury needs to reach a conclusion about 'what happened' based on the available evidence. Often this includes probabilistic evidence. Whereas Bayesian networks form a good tool for analysing evidence probabilistically, simply presenting the outcome of the network to a judge or jury does not allow them to make an informed decision. In this paper, we propose to combine Bayesian networks with a narrative approach to reasoning with legal evidence, the result of which allows a juror to reason with alternative scenarios while also incorporating probabilistic information. The proposed method aids both the construction and the understanding of Bayesian networks, using scenario schemes. We make three distinct contributions: (1) we propose to use scenario schemes to aid the construction of Bayesian networks, (2) we propose a method for producing scenarios in text form from the resulting networks and (3) we propose a format for reporting the alternative scenarios and their relations to the evidence (including strength).

A method for explaining Bayesian networks for legal evidence with scenarios

“…This way, the results of a network can be understood by a judge or jury in terms of narrative properties. This paper builds on previous work on the construction of Bayesian network graphs with scenarios (Vlek et al 2014), and extends previous preliminary ideas on explanation techniques using scenario schemes and scenario quality (Vlek et al 2015a, b). In this paper we combine and further formalise these preliminary ideas, and use them to propose a reporting format for explaining a Bayesian network.…”

“…Each scenario scheme idiom provides a fixed structure in which only elements of the scenario need to be filled in, and they enable the explanation of the resulting network. The concept of a scenario scheme idiom builds on the scenario idiom as proposed in Vlek et al (2014), but is now adapted based on preliminary work in Vlek et al (2015a). Three advantages of using scenario scheme idioms are as follows.…”